For many years, the mechanical door locks on automobiles were solely manual devices. The driver or passengers were required to manually actuate the mechanical door locks to change between the locked and the un-locked states. It is now common for automobiles to include the convenience of automatic door locks. A DC lock motor is provided in association with each mechanical door lock to actuate the mechanism for either locking or unlocking the door under the control of an electrical signal. A mechanical relay circuit is typically used to control the operation of the DC lock motor. One problem with relay control is that the relay is not capable of regulating or limiting current and thus excessive inrush or stall currents may occur within the relay when driving the DC lock motor. Indeed, current in a door lock system can reach, for example, 60 A and such a possible operating condition requires the use of heavier wiring in the wiring harness for the DC lock motor at an increased cost and weight penalty.
The mechanical relay circuit is typically of the single-pole double-throw (SPDT) switch type. The switch is normally switched to ground when not in use. With a ground connection in place, however, there are concerns with a fault mode in the wiring harness where the lock motor could be continuously activated through this ground connection. A potentially dangerous fire condition could arise from continuous actuation of the motor. To address this issue, the circuit designer will typically include a thermal limit protection device in the DC lock motor to sense an over-temperature condition, but this disadvantageously increases the cost of providing the automatic door lock convenience.
Another concern with mechanical relays is noise. A further concern with mechanical relays is jitter or bouncing.
In view of the foregoing, engineers have moved from mechanical relay circuit control to solid state relay circuit control. However, such solid state relays still suffer from the current and fault mode issues discussed above.
It is understood that even with automatic door locks, the driver or passengers may still desire to manually lock or unlock the lock mechanism. Detection of manual actuation of the lock is accordingly a desirable feature. As the lock mechanism is coupled to the DC lock motor, one way to accomplish this detection is to monitor the back electromotive force (BEMF) of the DC lock motor. However, with the relay switch connection to ground that is common practice in prior art designs the BEMF detection is not possible.
Modern automobiles are required to possess accident detection systems (and are now moving towards collision avoidance systems). Such systems are commonly used to trigger air bag deployment or other evasive action and may further be used to control other operations such as automatically contacting first responders and controlling the operation of the automatic door locks. With respect to the door locks, the common operation modes are to lock the doors in response to accident detection and then unlock the doors after the accident to permit first responders to enter the vehicle. Relay circuits, however, are inherently slow devices which cannot react in the less than a few hundred millisecond reaction time needed to ensure that the doors are locked in response to the detected accident.
In view of the foregoing, there is a need in the art for an improved automatic door lock motor driver circuit which would preferably be implemented in an integrated circuit format.